In general, signal processing refers to techniques and processes by which analog and digital signals are created, transmitted, received, and interpreted, among other functions. In many instances, the signals are electromagnetic signals that are processed using various electrical devices and circuits, some of which are known as transmitters, others of which are known as receivers. When the signals that are being transmitted and/or received are radio-frequency (RF) signals, the transmitters and receivers are known as RF transmitters and RF receivers.
In the context of a given transmitter, which may be part of a wireless communication device such as a mobile station, an access terminal, a cellular phone, and the like, it is often the case that a main controller of the device sends to the transmitter via internal circuit connections a digital signal for the transmitter to then process (e.g., modulate) and wirelessly transmit to a receiver, such as a base station for example. The output of the transmitter, then, is typically a signal onto which the digital signal from the main controller has been modulated. In the case of a type of modulation known in the art as amplitude modulation (AM), and in the case of wireless communications using a format known as time-division multiple access (TDMA), as examples, the digital signal from the controller is expressed in the signal emitted from the transmitter as a changing amplitude, which is sometimes referred to in the art as a changing output power envelope of the emitted RF signal.
A transmitter that is performing well emits an output signal having a changing output power envelope that is a close replica of the digital signal sent from the main controller to the transmitter. Some challenges to achieving a close replica that are faced by those in the art include system components (such as power amplifiers, specifically the output-power control thereof, and the like) and sensors (such as diodes and the like) exhibiting non-linear behavior, as well as the potentially distorting effects of phenomena such as changes in temperature and changes in frequency in a given power amplifier.
Those having skill in the relevant art will appreciate that elements in the figures are illustrated for simplicity and clarity, and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments. Furthermore, the apparatus and method components have been represented where appropriate by conventional symbols in the figures, showing only those specific details that are pertinent to understanding the disclosed embodiments so as not to obscure the disclosure with details that will be readily apparent to those having skill in the relevant art having the benefit of this description.